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sched/fair: Rework and comment the group_imb code
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Rik reported some weirdness due to the group_imb code. As a start to
looking at it, clean it up a little and add a few explanatory
comments.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-caeeqttnla4wrrmhp5uf89gp@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Peter Zijlstra authored and Ingo Molnar committed Sep 2, 2013
1 parent 6906a40 commit 30ce5da
Showing 1 changed file with 89 additions and 34 deletions.
123 changes: 89 additions & 34 deletions kernel/sched/fair.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4463,6 +4463,81 @@ fix_small_capacity(struct sched_domain *sd, struct sched_group *group)
return 0;
}

/*
* Group imbalance indicates (and tries to solve) the problem where balancing
* groups is inadequate due to tsk_cpus_allowed() constraints.
*
* Imagine a situation of two groups of 4 cpus each and 4 tasks each with a
* cpumask covering 1 cpu of the first group and 3 cpus of the second group.
* Something like:
*
* { 0 1 2 3 } { 4 5 6 7 }
* * * * *
*
* If we were to balance group-wise we'd place two tasks in the first group and
* two tasks in the second group. Clearly this is undesired as it will overload
* cpu 3 and leave one of the cpus in the second group unused.
*
* The current solution to this issue is detecting the skew in the first group
* by noticing it has a cpu that is overloaded while the remaining cpus are
* idle -- or rather, there's a distinct imbalance in the cpus; see
* sg_imbalanced().
*
* When this is so detected; this group becomes a candidate for busiest; see
* update_sd_pick_busiest(). And calculcate_imbalance() and
* find_busiest_group() avoid some of the usual balance conditional to allow it
* to create an effective group imbalance.
*
* This is a somewhat tricky proposition since the next run might not find the
* group imbalance and decide the groups need to be balanced again. A most
* subtle and fragile situation.
*/

struct sg_imb_stats {
unsigned long max_nr_running, min_nr_running;
unsigned long max_cpu_load, min_cpu_load;
};

static inline void init_sg_imb_stats(struct sg_imb_stats *sgi)
{
sgi->max_cpu_load = sgi->max_nr_running = 0UL;
sgi->min_cpu_load = sgi->min_nr_running = ~0UL;
}

static inline void
update_sg_imb_stats(struct sg_imb_stats *sgi,
unsigned long load, unsigned long nr_running)
{
if (load > sgi->max_cpu_load)
sgi->max_cpu_load = load;
if (sgi->min_cpu_load > load)
sgi->min_cpu_load = load;

if (nr_running > sgi->max_nr_running)
sgi->max_nr_running = nr_running;
if (sgi->min_nr_running > nr_running)
sgi->min_nr_running = nr_running;
}

static inline int
sg_imbalanced(struct sg_lb_stats *sgs, struct sg_imb_stats *sgi)
{
/*
* Consider the group unbalanced when the imbalance is larger
* than the average weight of a task.
*
* APZ: with cgroup the avg task weight can vary wildly and
* might not be a suitable number - should we keep a
* normalized nr_running number somewhere that negates
* the hierarchy?
*/
if ((sgi->max_cpu_load - sgi->min_cpu_load) >= sgs->load_per_task &&
(sgi->max_nr_running - sgi->min_nr_running) > 1)
return 1;

return 0;
}

/**
* update_sg_lb_stats - Update sched_group's statistics for load balancing.
* @env: The load balancing environment.
Expand All @@ -4475,15 +4550,12 @@ static inline void update_sg_lb_stats(struct lb_env *env,
struct sched_group *group, int load_idx,
int local_group, struct sg_lb_stats *sgs)
{
unsigned long nr_running, max_nr_running, min_nr_running;
unsigned long load, max_cpu_load, min_cpu_load;
struct sg_imb_stats sgi;
unsigned long nr_running;
unsigned long load;
int i;

/* Tally up the load of all CPUs in the group */
max_cpu_load = 0;
min_cpu_load = ~0UL;
max_nr_running = 0;
min_nr_running = ~0UL;
init_sg_imb_stats(&sgi);

for_each_cpu_and(i, sched_group_cpus(group), env->cpus) {
struct rq *rq = cpu_rq(i);
Expand All @@ -4495,16 +4567,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
load = target_load(i, load_idx);
} else {
load = source_load(i, load_idx);

if (load > max_cpu_load)
max_cpu_load = load;
if (min_cpu_load > load)
min_cpu_load = load;

if (nr_running > max_nr_running)
max_nr_running = nr_running;
if (min_nr_running > nr_running)
min_nr_running = nr_running;
update_sg_imb_stats(&sgi, load, nr_running);
}

sgs->group_load += load;
Expand All @@ -4522,21 +4585,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
sgs->group_power = group->sgp->power;
sgs->avg_load = (sgs->group_load*SCHED_POWER_SCALE) / sgs->group_power;

/*
* Consider the group unbalanced when the imbalance is larger
* than the average weight of a task.
*
* APZ: with cgroup the avg task weight can vary wildly and
* might not be a suitable number - should we keep a
* normalized nr_running number somewhere that negates
* the hierarchy?
*/
if (sgs->sum_nr_running)
sgs->load_per_task = sgs->sum_weighted_load / sgs->sum_nr_running;

if ((max_cpu_load - min_cpu_load) >= sgs->load_per_task &&
(max_nr_running - min_nr_running) > 1)
sgs->group_imb = 1;
sgs->group_imb = sg_imbalanced(sgs, &sgi);

sgs->group_capacity =
DIV_ROUND_CLOSEST(sgs->group_power, SCHED_POWER_SCALE);
Expand Down Expand Up @@ -4781,6 +4833,10 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
busiest = &sds->busiest_stat;

if (busiest->group_imb) {
/*
* In the group_imb case we cannot rely on group-wide averages
* to ensure cpu-load equilibrium, look at wider averages. XXX
*/
busiest->load_per_task =
min(busiest->load_per_task, sds->avg_load);
}
Expand All @@ -4798,6 +4854,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
if (!busiest->group_imb) {
/*
* Don't want to pull so many tasks that a group would go idle.
* Except of course for the group_imb case, since then we might
* have to drop below capacity to reach cpu-load equilibrium.
*/
load_above_capacity =
(busiest->sum_nr_running - busiest->group_capacity);
Expand All @@ -4813,11 +4871,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
* we also don't want to reduce the group load below the group capacity
* (so that we can implement power-savings policies etc). Thus we look
* for the minimum possible imbalance.
* Be careful of negative numbers as they'll appear as very large values
* with unsigned longs.
*/
max_pull = min(busiest->avg_load - sds->avg_load,
load_above_capacity);
max_pull = min(busiest->avg_load - sds->avg_load, load_above_capacity);

/* How much load to actually move to equalise the imbalance */
env->imbalance = min(
Expand Down Expand Up @@ -4881,7 +4936,7 @@ static struct sched_group *find_busiest_group(struct lb_env *env)

/*
* If the busiest group is imbalanced the below checks don't
* work because they assumes all things are equal, which typically
* work because they assume all things are equal, which typically
* isn't true due to cpus_allowed constraints and the like.
*/
if (busiest->group_imb)
Expand Down

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